The present invention relates to a magnetostatic field generator for a magnetic resonance imaging apparatus (abbreviated as an MRI apparatus hereinafter), and more particularly to a magnetostatic field generator for an MRI apparatus which is capable of generating an intense and uniform magnetostatic field over a wide range and is economical.
An MRI apparatus displays images of nuclear spin density distribution, relaxation time distribution and the like in an analyte as tomographical images by applying arithmetic processing to a signal measured by utilizing a nuclear magnetic resonance (NMR) phenomenon.
A magnetostatic field having an intensity and a direction which are uniform space-wise and time-wise is required for generating the NMR phenomenon. To be concrete, a magnetostatic field having an intensity of 0.04 to 2 tesla (T) and uniformity of about several ten ppm or less is required in a vacant space having a diameter of approximately 300 to 500 mm. Magnetostatic field generators for generating a magnetostatic field may be classified broadly into three types, i.e., that which uses permanent magnets, or those that use superconductive magnets and normal conductive magnets.
The present invention relates to a magnetostatic field generator using permanent magnets among these three types.
A perspective view of a magnetostatic field generator according to prior art is shown in FIG. 1. The details of the prior art is set forth in JP-A-62-177903. A reference numeral 51 in FIG. 1 represents a tubular core made of a soft magnetic material, and 60a, 60b, 61a, 61b, 62a and 62b represent permanent magnets. The permanent magnets 60a and 60b have a trapezoidal shape, respectively, and are fixedly attached to internal wall surfaces 65a and 65b perpendicular to a direction 100 of the uniform magnetic field of the core 51. Further, the direction of magnetization 70a and 70b is in the same direction as a direction 100 of uniform magnetic field. Furthermore, the permanent magnets 61a, 61b, 62a and 62b have a trigonal prism shape, respectively, and are fixedly attached to internal wall surfaces 66 and 67 parallel to the direction 100 of the uniform magnetic field of the core 51. Further, magnetization 71a, 71b, 72a and 72b of respective permanent magnets 61a, 61b, 62a and 62b point to direction perpendicular to boundary surfaces 81a, 81b, 82a and 82b facing to a vacant space 200.
With such a structure as described above, a uniform magnetic field is generated in the vacant space 200 surrounded by the permanent magnets 60a, 60b, 61a, 61b, 62a and 62b.
Here, a front view of a portion of only one quarter on the right upper side of FIG. 1 is shown in FIG. 2 for further detailed examination. As described above, a uniform magnetic field is generated by means of the permanent magnets 60a and 61a. In the vicinity 90 of a part where both magnets are in contact with each other, however, the permanent magnets are used only for passing a magnetic flux, and do not contribute to generation of the magnetic field in the vacant space 200. Namely, the rate of the intensity of a uniform magnetic field generated in the vacant space 200, i.e., a magnetic field generation efficiency, is lowered with respect to the weight of a permanent magnet material.
Since a permanent magnet material having a large value of a maximum energy product presently obtainable is very expensive, the ratio that the material cost thereof bears to the cost of the magnetic field generator is quite large. Accordingly, it becomes an important subject in development of a magnetic circuit to reduce the weight of the permanent magnets by improving the magnetic field generation efficiency.